Web ゲーム用のオーディオ

モバイルオーディオの注意点

By far the most difficult platforms to provide web audio support for are mobile platforms. Unfortunately these are also the platforms that people often use to play games. There are a couple of differences between desktop and mobile browsers that may have caused browser vendors to make choices that can make web audio difficult for game developers to work with. Let's look at these now.

自動再生

Many mobile browsers will simply ignore any requests made by your game to automatically play audio; instead playback for audio needs to be started by a user-initiated event. This means you will have to structure your audio playback to take account of that. This is usually mitigated against by loading the audio in advance and priming it on a user-initiated event.

For more passive audio auto play, for example background music that starts as soon as a game loads, one trick is to detect *any* user initiated event and start playback then. For other more active sounds that are to be used during the game we could consider priming them as soon as something like a start button is pressed.

To prime audio like this we want to play a part of it; for this reason it is useful to include a moment of silence at the end of your audio sample. Jumping to, playing and pausing that silence will mean we can now use JavaScript to play that file at arbitrary points.

音量

Programmatic volume control may be disabled in mobile browsers. The reason often given is that the user should be in control of the volume at the OS level and this shouldn't be overridden.

バッファリングと先読み

Likely as an attempt to mitigate runaway mobile network data use, we also often find that buffering is disabled before playback has been initiated. Buffering is the process of the browser downloading the media in advance, which we often need to do to ensure smooth playback.

並行したオーディオ再生

A requirement of many games is the need to play more than one piece of audio at the same time; for example, there might be background music playing along with sound effects for various things happening in the game. Although the situation is soon going to get better with the adoption of the Web Audio API, the current most widely-supported method — using the vanilla <audio> element — results in patchy results on mobile devices.

テストとサポート

Here's a table that shows what mobile platforms support the features talked about above.

モバイルの回避処理

Although mobile browsers can present problems, there are ways to work around the issues detailed above.

オーディオスプライト

Audio sprites borrow their name from CSS sprites, which is a visual technique for using CSS with a single graphic resource to break it into a series of sprites. We can apply the same principle to audio so that rather than having a bunch of small audio files that take time to load and play, we have one larger audio file containing all the smaller audio snippets we need. To play a specific sound from the file, we just use the known start and stop times for each audio sprite.

The advantage is that we can prime one piece of audio and have our sprites ready to go. To do this we can just play and instantly pause the larger piece of audio. You'll also reduce the number of server requests and save bandwidth.

var myAudio = document.createElement("audio");
myAudio.src = "mysprite.mp3";
myAudio.play();
myAudio.pause();

You'll need to sample the current time to know when to stop. If you space your individual sounds by at least 500ms then using the timeUpdate event (which fires every 250ms) should be sufficient. Your files may be slightly longer than they strictly need to be, but silence compresses well.

Here's an example of an audio sprite player — first let's set up the user interface in HTML:

<audio id="myAudio" src="https://jPlayer.org/tmp/countdown.mp3"></audio>
<button data-start="18" data-stop="19">0</button>
<button data-start="16" data-stop="17">1</button>
<button data-start="14" data-stop="15">2</button>
<button data-start="12" data-stop="13">3</button>
<button data-start="10" data-stop="11">4</button>
<button data-start="8"  data-stop="9">5</button>
<button data-start="6"  data-stop="7">6</button>
<button data-start="4"  data-stop="5">7</button>
<button data-start="2"  data-stop="3">8</button>
<button data-start="0"  data-stop="1">9</button>

Now we have buttons with start and stop times in seconds. The "countdown.mp3" MP3 file consists of a number being spoken every 2 seconds, the idea being that we play back that number when the corresponding button is pressed.

Let's add some JavaScript to make this work:

var myAudio = document.getElementById('myAudio');
var buttons = document.getElementsByTagName('button');
var stopTime = 0;

for (var i = 0; i < buttons.length; i++) {
  buttons[i].addEventListener('click', function() {
    myAudio.currentTime = this.getAttribute("data-start");
    stopTime = this.getAttribute("data-stop");
    myAudio.play();
  }, false);
}

myAudio.addEventListener('timeupdate', function() {
  if (this.currentTime > stopTime) {
    this.pause();
  }
}, false);

バックグラウンドミュージック(BGM)

Music in games can have a powerful emotional effect. You can mix and match various music samples and assuming you can control the volume of your audio element you could cross-fade different musical pieces. Using the playbackRate() method you can even adjust the speed of your music without affecting the pitch, to sync it up better with the action.

All this is possible using the standard <audio> element and associated HTMLMediaElement API, but it becomes much easier and more flexible with the more advanced Web Audio API. Let's look at this next.

ゲーム用のWebオーディオ API

Now that it's supported in all modern browsers except for Opera Mini and Internet Explorer (although Microsoft is now working on it), an acceptable approach for many situations is to use the Web Audio API (see the Can I use Web Audio API page for more on browser compatibility). The Web Audio API is an advanced audio JavaScript API that is ideal for game audio. Developers can generate audio and manipulate audio samples as well as positioning sound in 3D game space.

A feasible cross-browser strategy would be to provide basic audio using the standard <audio> element and, where supported, enhance the experience using the Web Audio API.

As the Web Audio API allows precise timing and control of audio playback, we can use it to play samples at specific moments, which is a crucial immersive aspect of gaming. You want those explosions to be accompanied by a thundering boom, not followed by one, after all.

Web Audio API でのBGM

Although we can use the <audio> element to deliver linear background music that doesn't change in reaction to the game environment, the Web Audio API is ideal for implementing a more dynamic musical experience. You may want music to change depending on whether you are trying to build suspense or encourage the player in some way. Music is an important part of the gaming experience and depending on the type of game you are making you may wish to invest significant effort into getting it right.

One way you can make your music soundtrack more dynamic is by splitting it up into component loops or tracks. This is often the way that musicians compose music anyway, and the Web Audio API is extremely good at keeping these parts in sync. Once you have the various tracks that make up your piece you can bring tracks in and out as appropriate.

You can also apply filters or effects to music. Is your character in a cave? Increase the echo. Maybe you have underwater scenes, so apply a filter that muffles the sound.

Let's look at some Web Audio API techniques for dynamically adjusting music from its base tracks.

トラックを読み込む

With the Web Audio API you can load separate tracks and loops individually using XMLHttpRequest, which means you can load them synchronously or in parallel. Loading synchronously might mean parts of your music are ready earlier and you can start playing them while others load.

Either way you may want to synchronize tracks or loops. The Web Audio API contains the notion of an internal clock that starts ticking the moment you create an audio context. You'll need to take account of the time between creating an audio context and when the first audio track starts playing. Recording this offset and querying the playing track's current time gives you enough information to synchronize separate pieces of audio.

To see this in action, let's lay out some separate tracks:

<ul>
  <li><a class="track" href="https://jPlayer.org/audio/mp3/gbreggae-leadguitar.mp3">Lead Guitar</a></li>
  <li><a class="track" href="https://jPlayer.org/audio/mp3/gbreggae-drums.mp3">Drums</a></li>
  <li><a class="track" href="https://jPlayer.org/audio/mp3/gbreggae-bassguitar.mp3">Bass Guitar</a></li>
  <li><a class="track" href="https://jPlayer.org/audio/mp3/gbreggae-horns.mp3">Horns</a></li>
  <li><a class="track" href="https://jPlayer.org/audio/mp3/gbreggae-clav.mp3">Clavi</a></li>
</ul>

All of these tracks are the same tempo and are designed to be synchronized with each other.

window.AudioContext = window.AudioContext || window.webkitAudioContext;

var offset = 0;
var context = new AudioContext();

function playTrack(url) {
  var request = new XMLHttpRequest();
  request.open('GET', url, true);
  request.responseType = 'arraybuffer';

  var audiobuffer;

  // Decode asynchronously
  request.onload = function() {
    
    if (request.status == 200) {
      
      context.decodeAudioData(request.response, function(buffer) {
        var source = context.createBufferSource();
        source.buffer = buffer;
        source.connect(context.destination);
        console.log('context.currentTime ' + context.currentTime);

        if (offset == 0) {
          source.start();
          offset = context.currentTime;
        } else {
          source.start(0,context.currentTime - offset);
        }

      }, function(e) {
        console.log('Error decoding audio data:' + e);
      });
    } else {
      console.log('Audio didn\'t load successfully; error code:' + request.statusText);
    }
  }
  request.send();
}

var tracks = document.getElementsByClassName('track');

for (var i = 0, len = tracks.length; i < len; i++) {
  tracks[i].addEventListener('click', function(e){

    playTrack(this.href);
    e.preventDefault();
  });
}

Now let's look over the code. First we set up a new AudioContext and create a function (playTrack()) that loads and starts playing a track.

start() (formerly known as noteOn()) will start playing an audio asset. start() asks three (optional) parameters:

1. when: The absolute time to commence playback.
2. where (offset): The part of the audio to start playing from.
3. how long: The duration to play for.

stop() takes one optional parameter — when — which is the delay before stopping.

If start()'s second parameter — the offset — is zero, we start playing from the start of the given piece of audio, which is what we do in the first instance. We then store the AudioContext.currentTime — the offset of when the first piece began playing, subtract that from any subsequent currentTimes to calculate the actual time, and use that to synchronize our tracks.

In the context of your game world you may have loops and samples that are played in different circumstances, and it can be useful to be able to synchronize with other tracks for a more seamless experience.

You may find that the introduction of a new track sounds more natural if it comes in on the beat/bar/phrase or whatever units you want to chunk your background music into.

To do this before playing the track you want to sync, you should calculate how long it is until the start of the next beat/bar etc.

Here's a bit of code that given a tempo (the time in seconds of your beat/bar) will calculate how long to wait until you play the next part — you feed the resulting value to the start() function with the first parameter, which takes the absolute time of when that playback should commence. Note the second parameter (where to start playing from in the new track) is relative:

if (offset == 0) {
  source.start();
  offset = context.currentTime;
} else {
  var relativeTime = context.currentTime - offset;
  var beats = relativeTime / tempo;
  var remainder = beats - Math.floor(beats);
  var delay = tempo - (remainder*tempo);
  source.start(context.currentTime+delay, relativeTime+delay);
}

場所のオーディオ

Positional audio can be an important technique in making audio a key part of an immersive gaming experience. The Web Audio API not only enables us to position a number of audio sources in three-dimensional space but can also allow us to apply filters that make that audio appear more realistic.

In short, using the positional capabilities of the Web Audio API we can relate further information about the game world to the player.

We can relate:

• The position of objects
• The direction of objects (movement of position and recreation of the Doppler effect)
• The environment (cavernous, underwater, etc.)

This is especially useful in a three-dimensional environment rendered using WebGL, where the Web Audio API makes it possible to tie audio to the objects and viewpoints.

こちらも確認

• Web Audio API on MDN
• <audio> on MDN
• Developing Game Audio with the Web Audio API (HTML5Rocks)
• Mixing Positional Audio and WebGL (HTML5Rocks)
• Songs of Diridum: Pushing the Web Audio API to Its Limits
• Making HTML5 Audio Actually Work on Mobile
• Audio Sprites (and fixes for iOS)